This invention relates to aqueous dispersions of particles of urea-aldehyde condensation products, which are useful as fertilizers, and to processes for preparing them.
Urea is commonly used as a source of nitrogen in fertilizers. It contains 46% by weight of available nitrogen, in readily water-soluble form. Because of its solubility, urea is often used in the form of prills, which are small globules of solidified molten urea, since the prills have a slow dissolution rate. However, in whatever form urea per se is commonly used, it is generally considered to be a quick release fertilizer. That is, its ready solubility means that it is carried into the soil rather quickly, and since it is in soluble form, it is absorbed rapidly by plant life.
Often today, fertilizers are formulated to release plant nutrients over a period of time, for more uniform effect on the plant life and to avoid the burning that sometimes occurs when too much fertilizer material is applied and becomes available within a very short space of time. To this end, urea has been applied to the soil in the form of urea-formaldehyde compositions. Because the urea is chemically bound, it releases nitrogen in available form, in which it can be assimilated by plant life, much less rapidly than urea per se.
The nitrogen in urea-formaldehyde fertilizer products is ordinarily characterized as: (1) soluble nitrogen, which may be derived from uncombined urea or other nitrogenous materials; (2) cold water insoluble nitrogen (CWIN); and (3) hot water insoluble nitrogen (HWIN). These latter two are generally referred to as water insoluble nitrogen (WIN). The methods for the determination of WIN, and the proportions thereof that are, respectively, CWIN and HWIN, are reported in the publication, Official Methods of Analysis of the Association of Official Agricultural Chemists, 11th Edition, 1970.
Liquid forms of urea-formaldehyde fertilizers, that can be applied by spraying, are commonly used by those offering commercial lawn care services. Often the formulations preferred by such commercial organizations provide a combination of a quick release nitrogen source, which may be urea, or a water-soluble urea-formaldehyde reaction product, for example, and a slow release nitrogen source, which generally is a water-insoluble urea-formaldehyde condensation product. Sources of phosphorus and potassium often are included in such formulations. Typically the formulation that is intended for application to park and residential lawns will provide for a sustained release of nitrogen during a period up to about 90 days. For other applications, such as the maintenance of turf areas on golf courses, longer sustained release periods may be preferred.
The production of a water-soluble urea-formaldehyde reaction product, suitable for fertilizer use, is described in U.S. Pat. No. 3,462,256. The product has less "burn" potential than urea, but is much more likely to "burn" than products produced in accordance with the present invention. The process comprises preparing an aqueous mixture of urea and formaldehyde having a urea:formaldehyde mol ratio above 1:1 but less than 2:1, adding ammonia in an amount of 0.3% to 6% by weight of the urea and formaldehyde, heating the mixture at a temperature between about 75.degree. C. and boiling while maintaining the pH of the mixture in the range of about 8.5 to 10 with strong alkali until at least about 90%. of the formaldehyde is in combined form, with at least 60% of the formaldehyde in the form of methylol compounds, and then continuing heating at pH 7 to 8.5 until at least 50%, but no more than 80%, of the formaldehyde is in the form of methylene groups.
The aqueous mixture of urea, formaldehyde and ammonia can be prepared from the individual components, i.e., a commercially available formaldehyde solution and urea in any convenient form. However, it can also be prepared by adding urea to an already partially condensed urea formaldehyde reaction product such as UF Concentrate 85 prepared in accordance with U.S. Pat. No. 2,652,377 and the term "aqueous mixture" mentioned above is meant to include mixtures prepared in this manner.
The Sartoretto et al. U.S. Pat. No. 4,298,512, also describes aqueous dispersions of urea-aldehyde polymers for use as liquid fertilizers. The urea-aldehyde polymers are primarily produced by the condensation of urea with formaldehyde, and the aldehyde content is modified by the inclusion in the reaction mixture of one or more alkyl aldehydes having from two to four carbon atoms. The nitrogen concentration in the dispersions described in the patent is generally on the order of from about 15% to about 17% by weight based on the total dispersion.
In the process of the Sartoretto patent, the formaldehyde source was generally either a water clear solution containing 60% formaldehyde and 25% urea by weight, referred to in the trade as a UFC-85 concentrate, or an aqueous formaldehyde stabilized with methanol and containing 37% formaldehyde by weight. A UFC-85 concentrate is a weakly reacted material. To prepare it, urea and formaldehyde are reacted together under mildly alkaline conditions in a manner similar to that described in U.S. Pat. No. 3,970,625. At the conclusion of the brief reaction period, the reaction product is concentrated to a high solids content of 85% by weight, hence the name, UFC-85. The product contains 60% formaldehyde by weight, 25% urea by weight (F/U molar ratio, 4.8 to 1), and a balance consisting of the aqueous vehicle. UFC-85 can also be made by continuous processes, such as, for example, by dissolving highly concentrated formaldehyde vapors, from a formaldehyde manufacturing process, in an aqueous solution of urea.
The 2-4 carbon alkyl aldehydes used in the Sartoretto patent as stabilizing reactants, that is, acetaldehyde, propionaldehyde, and butyraldehyde, are difficult materials to store and handle. Acetaldehyde, for example, has to be stored under refrigerated conditions and preferably under an atmosphere of nitrogen gas.
The reaction of urea and formaldehyde to form a stable dispersion of WIN particles is tricky because of the reactivity of these materials under acidic conditions. The reaction is exothermic and rapidly accelerates at elevated temperatures. The danger of run-away reactions resulting in a non-flowable or pasty condition is high. The use of a pre-reaction product, such as UFC-85, mitigates this reactivity to some extent.
Those skilled in the art of fertilizer manufacture are well aware of these and other problems concerning reaction control. The reaction is extremely sensitive, for example, to minor pH changes, which strongly influence the degree of precipitation of water insoluble nitrogen (WIN) particles. The pH also changes during the reaction--dropping initially and then rising as the reaction progresses. It is therefore very difficult to keep the reaction conditions constant and to exert any degree of accurate control over the reaction rate or the way in which the reaction progresses. Furthermore, there is no easily run test that can be used to determine the way in which the reaction is progressing. Control of the reaction therefore tends to be somewhat crude, relying upon time and temperature observations.
Since the urea-aldehyde condensation product that is formed during the reaction is insoluble in the aqueous vehicle, particles of the condensation product precipitate out. For optimum slow release, these particles should grow to be sufficiently large to be more aptly termed granules. If the reaction mixture is agitated, the particles tend to remain very fine in size. This in turn may result in an increase of viscosity as the reaction progresses.
Since the reaction goes forward at an elevated temperature, under either acid or alkaline conditions, although much more rapidly under acid conditions, the reaction must be stopped somehow, at some desired point, before a solid gel or an unworkable mass is formed by the reaction. Ordinarily, the reaction is stopped by cooling to ambient temperature and by adjusting the pH to neutral. Even so, the reaction may continue very slowly. Continuation of the reaction is extremely undesirable. For example, the liquid fertilizer composition may be placed in a storage tank, and if permitted to stand there for several weeks, may slowly gel in the tank to the point where it no longer flows.
The term "stability", as applied to a liquid fertilizer materials based on urea-formaldehyde condensation products, refers to the maintenance of the material in a readily flowable state. Stability is adversely affected when the WIN is present in very fine particle size, and also when the total amount of WIN particles present is below about 10% or above about 35% by weight based on the total nitrogen present.
The Michaud et al. U.S. Pat. No. 4,234,332, describes the use of dicyandiamide in an aqueous solution useful as a fertilizer. The presence of the dicyandiamide is said to enhance the solubility of nutrient materials present, improve stability against precipitation of ingredients from solution, and to inhibit nitrification after application of the fertilizer solution to the soil.